mention YAML in Markdown (withastro#3203)

src/content/docs/en/guides/markdown-content.mdx

@@ -7,7 +7,7 @@ i18nReady: true
 import Since from '~/components/Since.astro'
 import FileTree from '~/components/FileTree.astro'
 
-[Markdown](https://daringfireball.net/projects/markdown/) is commonly used to author text-heavy content like blog posts and documentation. Astro includes built-in support for standard Markdown files. 
+[Markdown](https://daringfireball.net/projects/markdown/) is commonly used to author text-heavy content like blog posts and documentation. Astro includes built-in support for standard Markdown files that can also include [frontmatter YAML](https://dev.to/paulasantamaria/introduction-to-yaml-125f) to define custom metadata such as a title, description, and tags.
 
 With the [@astrojs/mdx integration](/en/guides/integrations-guide/mdx/) installed, Astro also supports [MDX](https://mdxjs.com/) (`.mdx`) files which bring added features like support for JavaScript expressions and components in your Markdown content.